With the recent expansion of the military device spectrum into the millimeter wave region, a need has arisen for TWTs with unprecedently small bores, large energy products [greater than 30 megagauss-oersteds] and high intrinsic coercivities (greater than 12 kilo-oersteds). The foregoing considerations plus others have mitigated against the previously used Alnico-type magnets, and for the rare earth-cobalt (RECo) type magnets, particularly sumarium cobalt SmCo.sub.5 and Sm.sub.2 Co.sub.17 compositions. Also, differing magnetic configurations have been used in attempts to optimize the magnetic properties of the device(s).
Prior PPM stacks for lower frequency devices have used Alnico magnets of either axial or radial magnetic orientation to good effect. The reduction in size demanded by the designer of millimeter wave devices has, until recently, required the use of RECo magnets which are manufactured with only an axial magnetic orientation and alternated with pole pieces to conduct flux into the bore. This has led to extremely inefficient PPM stacks wherein the volume of the PPM stack bore was 1/20th of the total magnet material; a a 1-to-1 volume to bore ratio is considered in the range of optimum. A relatively new process for producing RECo magnets, called hot-isostatic-pressing process (HIP), has enabled them to be made very small and with a radial magnetic orientation which would not, by reason of stress cracks, fly apart upon release from its mold. A hybrid arrangement of axial and radial magnets is taught in U.S. Pat. No. 3,768,054 to W. Neugebauer. This device makes use of iron shells, pole pieces and large, unused, interior volumes: Further, the radially oriented magnets are arranged to surround the axially oriented magnets thus not leading to a teaching of the instant invention.
A growing need for extremely light-weight radars such as in remotely piloted vehicles (RPVs) has caused researchers to look to shorter and shorter wavelengths in order to solve their space and weight problems. However, existing amplifier tubes using Alnico magnets are not amenable to modification for small bores and short period magnetic circuits. They lack high coercivity and anisotropy necessary for direct contact PPM stacks. Fortunately, RECo magnets have these qualities and are almost immune to demagnetization and to change in magnetic orientation. Therefore, the problem existing in the art has been optimizing the design that will fulfill all the requirements.